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Effect of Doped NBT on Microstructure and Electrical of Barium Titanate-Based Lead-Free PTC Ceramic

Author: WuHaiDong
Tutor: PuYongPing
School: Shaanxi University of Science and Technology
Course: Materials Physics and Chemistry
Keywords: Positve temperature coefficient Microstructure barium titanate Na0.5Bi0.5TiO3 BiYO3
CLC: TM28
Type: Master's thesis
Year: 2012
Downloads: 76
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Abstract


Nowadays, BaTiO3-based PTC ceramics with high Curie temperaturemostly contain lead. However, lead is volatile at high temperature, which doesharm to people during the process of producing and using ceramics. Taking allthese into account, it is urgert to development lead-free PTC （PositiveTemperature Coefficient） ceramcis with high Curie temperature, low resisitivityroom temperature and obvious resistance jump. In order to substitute thelead-contained PTC ceramics, three kinds of BaTiO3-based lead-free PTCceramic system were investigated in the thesis.In this paper, BaTiO3-Na0.5Bi0.5TiO3, BaTiO3-BiYO3and BaTiO3-Na0.5Bi0.5TiO3-BiYO3system ceramics were fabricated by the conventionalmixed oxide method. The results showed that the ceramic samples doped Nb2O5could obtain good PTC effect （ρ max ρmin>103）, low room temperature resistivity（ρRT103cm） when the content of dopant （Na0.5Bi0.5TiO3） was less than2mol%, but it was difficult to fulfill the request of the Curie point in practicalapplications. When the content of dopant exceeded2mol%, the ρRTreached106cm, so it was hard to be semiconducting for the ceamic samples. Therefore,BaTiO3-BiYO3and BaTiO3-Na0.5Bi0.5TiO3-BiYO3system ceramics werefabricated by doping donors such as Nb2O5. In addition, the samples werecharacterized by X-ray diffraction, scanning electron microscope, impedancespectroscopy and the temperature dependence of resistivity, and the effects ofcomposition and preparation process on the PTC characteristics andmicrostructure were investigated.In order to reduce the room temperature resistivity and enhance theresistance jump, Nb2O5as donor and MnCO3as acceptor were introduced intoBaTiO3-Na0.5Bi0.5TiO3ceramics，and the semiconducting ceramics could beobtained by doping Nb2O5, then the influence of doping Nb5+and Mn2+on thePTC effects and microstructure of BaTiO3-Na0.5Bi0.5TiO3ceramics were stuied. The research results showed that the room temperature resistivity could besignificantly reduced with the addition of Nb5+and the resistance jump can beenhanced with the addition of Mn2+, but the Curie temperature was shifted to alower temperature with increasing of the content of Nb5+. In addition, theaverage grain size of samples decreased with increasing of the contents of donorNb5+and acceptor Mn2+. However, the inhibitory extent of donor Nb5+on thegrain growth was much greater than that of acceptor Mn2+.In order to search for new system ceramics with higher Curie temperature,we studied BaTiO3-BiYO3system ceramics. The semiconducting ceramics couldbe obtained by doping Nb2O5, and the influence of doping BiYO3on the PTCeffects and microstructure of BaTiO3-based ceramics were stuied. The researchresults showed that the Curie temperature was shifted to a higher temperaturewith increasing of the content of BiYO3. The0.97BaTiO3-0.03BiYO3-0.002Nb2O5ceramics could obtain lowρRT（4000cm）, high Curie temperature（150℃） and good PTC effect （ρ max ρ>103min）. In addition, the average grainsize decreased with increasing of BiYO3.In order to improve the Curie point significantly, Na0.5Bi0.5TiO3and BiYO3as another end member of BaTiO3-based solid solutions were addedsimultaneously, then the influence of doping Na0.5Bi0.5TiO3and BiYO3on thePTC effects and microstructure of BaTiO3-based ceramics were stuied. Theresearch results showed that the obtained0.88BaTiO3-0.10Na0.5Bi0.5TiO3-0.02BiYO3ceramics prepared by a reduction-reoxidation exhibited a high Curietemperature of162℃. In addition, the impedance spectra of BaTiO3-Na0.5Bi0.5TiO3-BiYO3ceramic samples were measured from the roomtemperature to180℃. The results demonstrated that the resistivity of grainboundaries took the dominant position in PTC effect. The resistivity of grainboundary quickly increased with increasing of the contents of Na0.5Bi0.5TiO3andBiYO3, but the change of the resistivity of grains was very little.

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